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Intermittent G-K shorts are where the "octopus" 2-terminal curve tracer/component tester really pays for itself. Even a brief contact between a loose grid wire and the filament will 'flip' the trace on the 'scope screen.

Probably the most insidious thing to go wrong with a 3-500Z, and way too common. The control grid is a cylindrical "fence" of parallel vertical wires held in place by a wire ring at the top, bottom and middle of the structure. When the spot weld holding one end of a grid wire comes loose from the ring at that end, no problem occurs until heat warps the wire. The loose end will "curl" away from the broken weld spot until it touches the filament, or worse, the anode.

Driving the tube too hard, or loading the output too tightly can push grid current high enough to overheat the grid and break one of the spot welds.

Our routine test before we'll power up a 3-500Z in the customer's equipment is to hook the octopus from one cathode to one grid pin and (gently) slap the tube on the side, turning it right-side up, upside-down and sideways. Any flash of movement on the 'scope screen means that tube will be toxic to the amplifier sooner or later.

73

<blockquote data-quote="nomadradio" data-source="post: 607033" data-attributes="member: 22">Intermittent G-K shorts are where the &quot;octopus&quot; 2-terminal curve tracer/component tester&nbsp; really pays for itself. Even a brief contact between a loose grid wire and the filament will &#039;flip&#039; the trace on the &#039;scope screen.Probably the most insidious thing to go wrong with a 3-500Z, and way too common. The control grid is a cylindrical &quot;fence&quot; of parallel vertical wires held in place by a wire ring at the top, bottom and middle of the structure. When the spot weld holding one end of a grid wire comes loose from the ring at that end, no problem occurs until heat warps the wire. The loose end will &quot;curl&quot; away from the broken weld spot until it touches the filament, or worse, the anode.Driving the tube too hard, or loading the output too tightly can push grid current high enough to overheat the grid and break one of the spot welds.Our routine test before we&#039;ll power up a 3-500Z in the customer&#039;s equipment is to hook the octopus from one cathode to one grid pin and (gently) slap the tube on the side, turning it right-side up, upside-down and sideways. Any flash of movement on the &#039;scope screen means that tube will be toxic to the amplifier sooner or later.73</blockquote>

[QUOTE="nomadradio, post: 607033, member: 22"] Intermittent G-K shorts are where the "octopus" 2-terminal curve tracer/component tester really pays for itself. Even a brief contact between a loose grid wire and the filament will 'flip' the trace on the 'scope screen. Probably the most insidious thing to go wrong with a 3-500Z, and way too common. The control grid is a cylindrical "fence" of parallel vertical wires held in place by a wire ring at the top, bottom and middle of the structure. When the spot weld holding one end of a grid wire comes loose from the ring at that end, no problem occurs until heat warps the wire. The loose end will "curl" away from the broken weld spot until it touches the filament, or worse, the anode. Driving the tube too hard, or loading the output too tightly can push grid current high enough to overheat the grid and break one of the spot welds. Our routine test before we'll power up a 3-500Z in the customer's equipment is to hook the octopus from one cathode to one grid pin and (gently) slap the tube on the side, turning it right-side up, upside-down and sideways. Any flash of movement on the 'scope screen means that tube will be toxic to the amplifier sooner or later. 73 [/QUOTE]

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